Add native return type to ListLookup::getTimestamp() #6584

Problem

ListLookup::getTimestamp() has no native return type. The interface phpdoc says @return int, but the actual implementations disagree:

• 4 SQL implementations (PropertyUsageListLookup, UndeclaredPropertyListLookup, UnusedPropertyListLookup, UsageStatisticsListLookup) return wfTimestamp(TS_UNIX) which is string|false
• CachedListLookup stores the result in a private int $timestamp property, so PHP silently casts the string to int on assignment, and returns int

The implementations' own phpdoc says @return string|false or @return false|string, contradicting the interface's @return int.

Callers

All callers pass the return value to Language::userTimeAndDate() or Language::timeanddate(), which internally call wfTimestamp() and accept any timestamp format — so both string and int work fine in practice.

Options

1. int — Cast in each SQL implementation with (int)wfTimestamp(TS_UNIX). Matches what callers receive through CachedListLookup. Natural type for Unix timestamps.
2. string|false — Match what wfTimestamp returns. Change CachedListLookup::$timestamp to string|false. But false can never actually occur (current-time timestamp never fails).

Context

The other three ListLookup methods (fetchList(): array, isFromCache(): bool, getHash(): string) were typed in #6583.